ThermoCalc.java

import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class ThermoCalc {
    private JPanel panel1;
    private JComboBox ThermoOp;
    private JTextField tfValue;
    private JTextField tfResult;
    private JButton btnReset;
    private JButton btnCalculate;
    private JButton btnExit;

    public static void main(String[] args) {
        JFrame frame = new JFrame("Thermo Calculator");
        frame.setContentPane(new ThermoCalc().panel1);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }

    public ThermoCalc() {
        String[] conversions = {"Celcius to Reamur", "Celcius to Fahrenheit", "Celcius to Kelvin", "Reamur to Celcius",
                "Reamur to Fahrenheit", "Reamur to Kelvin", "Fahrenheit to Celcius", "Fahrenheit to Reamur", "Fahrenheit to Kelvin",
                "Kelvin to Celcius", "Kelvin to Reamur", "Kelvin to Fahrenheit"};
        ThermoOp.setModel(new DefaultComboBoxModel<>(conversions));

        btnReset.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                tfValue.setText("");
                tfResult.setText("");
                tfValue.requestFocus();
            }
        });

        btnExit.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                System.exit(0);
            }
        });

        btnCalculate.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                String selectedOp = (String) ThermoOp.getSelectedItem();

                double Value;
                try {
                    Value = Double.parseDouble(tfValue.getText());
                } catch (NumberFormatException ex) {
                    JOptionPane.showMessageDialog(null, "Please enter a valid number.");
                    return;
                }

                double result = 0;
                switch (selectedOp) {
                    case "Celcius to Reamur":
                        result = CtR(Value);
                        break;
                    case "Celcius to Fahrenheit":
                        result = CtF(Value);
                        break;
                    case "Celcius to Kelvin":
                        result = CtK(Value);
                        break;
                    case "Reamur to Celcius":
                        result = RtC(Value);
                        break;
                    case "Reamur to Fahrenheit":
                        result = RtF(Value);
                        break;
                    case "Reamur to Kelvin":
                        result = RtK(Value);
                        break;
                    case "Fahrenheit to Celcius":
                        result = FtC(Value);
                        break;
                    case "Fahrenheit to Reamur":
                        result = FtR(Value);
                        break;
                    case "Fahrenheit to Kelvin":
                        result = FtK(Value);
                        break;
                    case "Kelvin to Celcius":
                        result = KtC(Value);
                        break;
                    case "Kelvin to Reamur":
                        result = KtR(Value);
                        break;
                    case "Kelvin to Fahrenheit":
                        result = KtF (Value);
                        break;
                    default:
                        break;
                }
                tfResult.setText(String.valueOf(result));
            }
        });
    }

    public double CtR(double value) {
        return value * (4.0/5.0);
    }

    public double CtF(double value){
        return (value * 9.0/5.0) + 32.0;
    }

    public double CtK(double value){
        return  value + 273.15;
    }

    public double RtC(double value){
        return value * 5.0/4.0;
    }

    public double RtF(double value){
        return (value*9.0/4.0) + 32.0;
    }

    public double RtK(double value){
        return (value*5.0/4.0) + 273.15;
    }

    public double FtC(double value){
        return (value - 32.0) * 5.0/9.0;
    }

    public double FtR(double value){
        return (value -32.0) * 4.0/9.0;
    }

    public double FtK(double value){
        return (value-32.0) * 5.0/9.0 + 273.15;
    }

    public double KtC(double value){
        return value-273.15;
    }

    public double KtR (double value){
        return (value-273.15) * 4.0/5.0;
    }

    public double KtF(double value){
        return (value-273.15) * 9.0/5.0 +32.0;
    }

}